The present invention relates to an electrode for polarographic measurement of the partial pressure of oxygen in gases or solutions.
Electrodes of this kind, also called "Clark electrodes," are known and used in various technical fields, inter alia for determining the partial pressure of oxygen in blood, but also for determining the partial pressure of oxygen in other liquids and in gases.
These electrodes comprise a cathode in contact with and outwardly (in other words, in the direction of the medium to be measured) shielded by a gas permeable membrane, a half cell serving as an anode, this being a silver/silver halide half cell, usually simply a silver strip covered with silver halide, and a halogen-containing aqueous electrolyte which is in contact with the cathode and the anode. In the electrodes of this type used in practice, the halide is chloride.
When using such an electrode for polarographic oxygen determination, a constant potential is applied between the anode and the cathode. Oxygen from the solution or the gas to be measured diffuses through the membrane and is reduced to hydrogen peroxide and/or water on the cathode, which results in an electrical current, the magnitude of which is dependent upon the O.sub.2 -concentration in the measured medium. By means of suitable current measuring equipment of known type, this electrical current is measured, and the oxygen partial pressure in the measured medium is calculated on the basis of the current measurement; in practice, the current measuring equipment is usually so calibrated that the calculted oxygen pressure corresponding to the current can be directly read in mm Hg.
In the known art electrodes of this type in which the anode is an Ag/AgCl half cell and the electrolyte usually contains NaCl in a concentration of about 0.13M, the optimum polarisation voltage will be -630 mV, in other words, the voltage of the cathode is -630 mV, in relation to the voltage of the anode, as it has been found that the best linear relation between O.sub.2 concentration in the measured medium and the polarisation current is obtained at this polarisation voltage. At higher or lower polarisation voltages, the current will vary in a more or less non-linear manner. Therefore, for the major part, existing apparatus for O.sub.2 measurement using oxygen electrodes of the type here described, for example, for use in connection with micro-sized oxygen electrodes for measuring the partial pressure of oxygen in blood, is adapted to work with a polarisation voltage of -630 mV.
In electrodes of the type mentioned, the magnitude of the electrical current measured is also dependent upon the area of the cathode. Unfortunately, the cathode area in the existing electrodes has been found to increase continuously during the use of the electrode, which is due to the fact that silver from the half cell serving as the anode is being dissolved in the electrolyte and from there reduced out as free silver on the cathode. Naturally, this incurs an undesirable drift of the electrode response on given partial pressures of O.sub.2 in the measured medium and, therefore, constitutes a considerable source of measurement inaccuracy, especially when the cathode is small, for example, smaller than about 100 .mu., such as is often the case in modern electrodes for the measurement of the partial pressure of oxygen in blood.